Following completion of the 2000-2001 design, two flaws were discovered but could not be addressed before the completion of the academic year. First, the belt driving the shoulder twist pulley was found to slip. It appeared that the Neoprene belt stretched under the stress of the pulley system. Second, binding occurred between the elbow gearing due to a lack of thrust support for both sides of the worm. Additionally, the two-part aluminum elbow shaft was too weak to support the forearm. The aluminum yielded from the stress of the steel pin securing the elbow worm gear to the shaft.

To eliminate slipping at the shoulder twist pulley, a stranded or V-shaped belt should replace the Neoprene belt. Because a stranded or V-shaped belt is less likely to stretch under tension, the belt will maintain high pressure around the pulley, eliminating slip.

To eliminate binding, components for the elbow were redesigned. The steel drive shaft for the screwdriver motor is elongated to enable placing a 0.434-inch stainless steel thrust bearing at the free end. A small aluminum block houses the thrust bearing and mounts within the lower arm tubing. The elbow motor is repositioned to accommodate the additional thrust bearing. See Figure AD.1 for the cutaway view of the elbow joint incorporating the thrust bearing.

Text Box: Figure AD.1 -  Cutaway View of Redesigned Elbow Joint


Manufacturing the two-part elbow shaft design with steel is not possible because steel cannot be machined to the tolerances required for the design. Therefore, the two-part elbow shaft is replaced with a solid 0.25-inch diameter shaft. The repositioned motor requires the elbow shaft and worm gear to be offset within the lower arm tube. Finally, the elbow brackets were revised to still permit the forearm tubing to rest upon the lower arm tubing in the home position. Please see Figure AD.2 for the final elbow redesign.

Text Box: Figure AD.2  -  Final Redesigned Elbow


While the design changes have not been evaluated for their feasibility, experimental performance during the final presentation for the 2000-2001 arm indicates that the modifications will adequately support the forearm. Since bearing support and the resizing of the elbow brackets were the only major modifications, the modifications will eliminate the flaws and produce a successful design.


Table AD.1 Elbow Revision Bill of Materials




Part Number





PIC Design